Enhanced image data/dose reduction

ABSTRACT

A method includes generating enhanced image data based on lower dose image data and a predetermined image quality threshold, wherein an image quality of the enhanced image data is substantially similar to an image quality of higher dose image data, and a system includes an image quality enhancer ( 128 ) that generates enhanced image data based on lower dose image data and a predetermined image quality threshold, wherein an image quality of the enhanced image data is substantially similar to an image quality of higher dose image data.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application Ser.No. 61/264,340 filed Nov. 25, 2009 and U.S. provisional application Ser.No. 61/286,477 filed Dec. 15, 2009, both of which is incorporated hereinby reference.

The following generally relates to enhancing image data such asenhancing image quality (reducing noise) of lower dose image data to besubstantially the same as the image quality of higher dose image data,and finds particular application to computed tomography (CT). However,it also amenable to other medical imaging applications and tonon-medical imaging applications.

A multi-slice computed tomography (CT) scanner includes an x-ray tubemounted on a rotatable gantry that rotates around an examination regionabout a longitudinal or z-axis. The x-ray tube emits radiation thattraverses the examination region and a subject or object therein. Atwo-dimensional detector array subtends an angular arc opposite theexamination region from the x-ray tube. The detector array includes aplurality of rows of detectors that are aligned with respect to eachother and that extend along the z-axis. The detectors detect radiationthat traverses the examination region and the subject or object thereinand generate projection data indicative thereof. A reconstructorprocesses the projection data and reconstructs three-dimensional (3D)volumetric image data indicative thereof. The volumetric image data isprocessed to generate one or more images of the examination region,including the portion of the subject or object disposed therein.

The literature indicates that an estimated 68.7 million CT procedureswere performed in the U.S. in 2007. Unfortunately, CT scanners emitionizing radiation and thus expose the patient to ionizing radiation.The radiation dose deposited in the patient depends on multiple factors,including, but not limited to, tube current (mAs), tube voltage (kVp),pitch/exposure time (for helical scans), slice thickness and spacing(for axial scans), the number of scans in a study, and patient build.The dose deposited in the patient can be reduced by decreasing tubecurrent, tube voltage and/or the number of scans, and/or increasing thepitch, slice thickness and/or slice spacing. However, image noise isinversely proportional to radiation dose, and thus reducing radiationdose not only reduces the dose deposited in the patient but alsoincreases image noise, which reduces image quality (e.g., imageresolution), which may decrease the diagnostic value of the procedure.

Aspects of the present application address the above-referenced mattersand others.

According to one aspect, a method includes generating enhanced imagedata based on lower dose image data and a predetermined image qualitythreshold, wherein an image quality of the enhanced image data issubstantially similar to an image quality of higher dose image data.

According to another aspect, a system includes an image quality enhancerthat generates enhanced image data based on lower dose image data and apredetermined image quality threshold, wherein an image quality of theenhanced image data is substantially similar to an image quality ofhigher dose image data.

According to another aspect, a method includes determining a noisestandard deviation for the image data, a first set of outliers in theimage data based on the noise standard deviation and a global algorithm,and a noise distribution based on a set of inliers, which are determinedbased on the first set of outliers.

According to another aspect, a method includes fitting one or moremodels to the image data, identifying a model of the one or more modelsas likely to remove noise from the image data while preserving structurerepresenting a scanned object or subject in the image data, andemploying the identified model to remove noise from the image data andgenerate noise reduced image data.

According to another aspect, a method includes obtaining lower doseimage data, obtaining noise reduced image data, wherein the noisereduced image data is generated based on the lower does image data orprojection data used to generated the lower dose image data, andgenerating enhanced image quality image data based on the lower doseimage data and the noise reduced image data.

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating the preferred embodiments and arenot to be construed as limiting the invention.

FIG. 1 illustrates an imaging system and an image quality enhancer.

FIG. 2 illustrates an example of the image quality enhancer thatgenerates enhanced image data based on image data.

FIG. 3 illustrates a noise modeler of the image quality enhancer.

FIG. 4 illustrates a noise remover of the image quality enhancer.

FIG. 5 graphically illustrates gradient magnitude curves for full dose,reduced dose, and reduced dose with enhancement scans.

FIG. 6 illustrates an example of the image quality enhancer thatgenerates enhanced image data based on projection data.

FIGS. 7-11 illustrate various methods.

The following generally relates to enhancing or reducing noise in lowerdose image data based on a predetermined image quality (noise) thresholdso that the image quality of the enhanced lower dose image data issubstantially similar to the image quality of higher dose image data.This allows for reducing patient dose while retaining image quality.

FIG. 1 illustrates an imaging system 100 such as a computed tomography(CT) scanner. The imaging system 100 includes a generally stationarygantry 102 and a rotating gantry 104. The rotating gantry 104 isrotatably supported by the stationary gantry 102 and rotates around anexamination region 106 about a longitudinal or z-axis 108.

A radiation source 110, such as an x-ray tube, is supported by therotating gantry 104. The radiation source 110 emits radiation from afocal spot and the radiation traverses the examination region 106. Asource controller 112 controls the source 110, including source currentand/or voltage.

A source collimator 114 includes collimation members that collimate theradiation to form a generally cone, wedge, fan or other shaped radiationbeam. A collimator controller 116 controls the source collimator 114,including the relative position of the members, which facilitatesdefining x and/or z-axis radiation beam width.

A two-dimensional radiation sensitive detector array 118 subtends anangular arc opposite the radiation source 110 across the examinationregion 106. The detector array 118 includes a plurality of rows ofdetectors that extend along the z-axis 108 direction. The detector array118 detects radiation traversing the examination region 106 andgenerates projection data indicative thereof.

A reconstructor 120 reconstructs the projection data and generatesthree-dimensional (3D) volumetric image data indicative thereof. Thereconstructor 120 may employ a conventional 3D filtered-backprojectionreconstruction, a cone beam algorithm, an iterative algorithm and/orother algorithm.

A patient support 122, such as a couch, supports an object or subjectsuch as a human patient in the examination region 106. A patient supportcontroller 124 controls the patient support 122, including a speed atwhich the patient support 122 moves through examination region 106during scanning.

A general-purpose computing system or computer serves as an operatorconsole 126. A processor of the console 126 executes computer readableinstructions on the console 126, which allows an operator to controloperation of the system 100 such as selecting a low dose scan protocol,setting scan parameters such as source current, source voltage, beamwidth, pitch, slice thickness, type of scan (e.g., axial orhelical/spiral) and, number of scans, initiating scanning, etc.

The system 100 can be used for performing full dose scans and lower orreduced dose scans.

An image quality enhancer 128 enhances image data generated by theimaging system 100 (or other imaging system) such as image data from alower dose scan. In one embodiment, the image data is enhanced based ona predetermined image quality threshold such as an image noisethreshold. As described in greater detail below, this may includedetermining a noise standard deviation for the image data or a noisevariance distribution from the projection data used to generate theimage data, and generating enhanced image data based on the initiallower dose image data and noise reduced image data generated with thenoise standard deviation or noise variance distribution.

As a result, the image enhancer 128 can be used to enhance the imagequality of image data for a lower dose scan to a level that is about thesame or substantially similar to the image quality in an equivalenthigher dose or full dose scan. Hence, patient dose can be reduced whileretaining a predetermined level of image quality and preserving theunderlying structure of the scanned object. The image enhancer 128 canfurther be configured to enhance particular structure of interest (e.g.,edges) and/or generate data that is substantially invariant to one ormore of the acquisition parameters (e.g., slice thickness).

In the illustrated embodiment, the image enhancer 128 is part of thesystem 100, but separate from the console 126. In another embodiment,the image enhancer 128 is part of the console 126. In yet anotherembodiment, image enhancer 128 is located remote from the system 100,for example, in a computing system such as a workstation outside of theexam room in which the system 100 is located. The image enhancer 128also includes one or more processors that execute one or more computerreadable and executable instructions stored or encoded in computerreadable storage medium of the enhancer 128 or other component toperform the functionality discussed herein.

FIG. 2 illustrates an example of the image quality enhancer 128. In thisexample, the image enhancer 128 is configured to enhance image databased on a noise distribution determined from reconstructed (image)data.

The image enhancer 128 includes an acquisition parameter analyzer 202that analyzes one or more of the data acquisition (scan) parameters usedto scan an object or subject and generate the image data. It is to beappreciated that the acquisition parameters can be obtained from theconsole 126, memory, and/or other component, and/or manually entered bya user.

The acquisition parameter analyzer 202 automatically determinesinformation from the data acquisition parameters, and the informationcan be taken into account when generating the enhanced image data. Byway of example, the acquisition parameter analyzer 202 can automaticallyidentify information from the data acquisition parameters related slicethickness, and this information can be used to generate enhanced imagedata that is substantially invariant to the acquisition slice thickness.

The information can be taken into account when determining the spatialkernel weights defined below (e.g., in Equation 2) and used for noiseremoval and includes, but is not limited to, a minimum number of spatialweights and a central spatial weight for filtering kernel. These twoparameters may facilitate balancing a relative influence of neighboringvoxels in the noise removal kernel while keeping the size and strengthof the spatial component of the filtering kernel invariant to the studyslice thickness. This balance leads to results that are close to beinginvariant to the acquisition slice thickness. Once the spatial weightingkernel is set, slice thickness related algorithm parameters are derivedfrom it.

It is to be understood that other information can additionally oralternatively be obtained from the data acquisition parameters and usedto produce the enhanced image data. Moreover, in another embodiment, theacquisition parameter analyzer 202 is omitted. In such an embodiment,the particular information is otherwise determined and conveyed to theimage quality enhancer 128 or is not used in determining the enhancedimage data.

The image enhancer 128 further includes a noise modeler 204 that modelsor calculates a noise distribution based on the image data. An exampleof such a noise modeler is illustrated in FIG. 3.

As shown in FIG. 3, the noise modeler 204 includes a noise standarddeviation (STD) determiner 302 that determines a noise standarddeviation for sub-portions of voxels in the image data. Generally, thestandard deviation of a sub-portion of voxels behaves as a randomvariable with a mixture density distribution, as the sub-portions varyacross the image volume. These sub-portions include outliers (edges)corresponding to non-homogeneous regions and sub-portions that includeinliers (non-edges) corresponding to homogenous regions.

A global outlier identifier 304 identifies outliers in the image databased on a global algorithm and the noise standard deviation. In onenon-limiting instance, this includes segmenting the scanned object orsubject from the rest of the image data. The segmentation can beachieved through thresholding followed by a region growing technique orother technique. Segmenting the data may facilitate reducing the numberof voxels that will subsequently be processed and thus reduce processingtime.

The global outlier identifier 304 then analyzes the segmented data. Inone instance, the global analysis includes using a histogram of thelocal STD of small regions of the scanned object or subject in which thehistogram represents the distribution of the local STD in the imagedata. The local STD on suspected outliers can be identified using thishistogram, for example, as the signal values past a fixed percentage ofthe distribution or otherwise.

An inlier processor 306 processes the inliers (determined from theoutliers) and generates the noise STD distribution for the image data.In the illustrated embodiment, the inlier processor 306 processes theinliers based on a smoothing algorithm such as a wide two-dimensional(2D) median filter algorithm, which is applied over local STD imagedata. In the illustrated embodiment, values in a resulting STD matrixthat are designated as outliers are ignored, and any remaining outliersin the new noise level estimate are filled using interpolation (e.g.,linear, spline, etc.).

In an alternative embodiment, the wide median is approximated using amulti resolution technique. Using an approximation may facilitatedecreasing processing time. In another alternative embodiment, theoutlier local STD values are replaced with a repeated pattern thatincludes the two extreme values of the local STD distribution, and thepattern is synchronized with the multi resolution calculations. Thisreplacement allows fast calculation of a conventional median on regionsin an image for each separate resolution. The specific replacementpattern can be selected so that the approximated result is very close tothe result of a regular median calculation.

A decision component 308 determines whether to refine the noise STDdistribution. In the illustrated embodiment, this decision is based atleast in part on predetermined decision criteria 310, which can bestored in local or remote memory. In one instance, the criteria includesa maximum number of iterations. Additionally or alternatively, thecriteria is based on a difference between the results of two iterationsand a predetermined difference range, wherein a subsequent iteration isnot performed if the difference is within the predetermined differencerange. Additionally or alternatively, the criteria is based on apredetermined time period, wherein a subsequent iteration is notperformed after the time period lapses.

For instances in which the noise STD distribution is to be refined, alocal outlier and/or inlier identifier 312 identifies a set of outliersfrom the inliers based on a local algorithm. In one instance, thisincludes adjusting the set of outliers based on a comparison between alocal standard deviation of the original image data and the new noiselevel estimate. This can be achieved by identifying a local standarddeviation in the original image as an outlier only if its value issignificantly higher than its corresponding local noise level estimate.Then a new estimate is determined for the local noise level as discussedabove.

The noise modeler 204 outputs a signal indicative of the noise STDdistribution.

The image quality enhancer 128 further includes a noise remover 206 thatremoves noise from the image data based on the noise STD distribution.An example of a noise remover 206 is illustrated in FIG. 4.

As shown in FIG. 4, the noise remover 206 includes a model fitter 402that fits various local structural models 404 from a model bank 406 tothe image data. In one embodiment, this includes fitting one or morelocal structure models to each voxel and its three-dimensional (3D)neighborhood.

The noise remover 206 also includes a model selector 408 that selects amodel from the fitted models that is likely to remove noise from theimage data while preserving the structure representing the scannedobject or subject in the image data. The illustrated model selector 408selects a model based on one or more algorithms 410 such a statisticallearning or other machine learning algorithm in an algorithm bank 412.

The noise remover 206 also includes a model applier 414 that applies theselected model to the image data to remove noise. A new estimated valueof the voxel is determined from a corresponding model value. The outputof the model applier 414 is denoised image data.

The following describes a non-limiting example of fitting models,selecting a model, and applying the selected model. For this example,given a 3D image volume V of voxels with indices i, j and k, a noiselessstructure M that has the smallest weighted Euclidian distance from thesub-volume V_(i+i′,j+j′,k+k′) of the noisy image V can be found based onEquation 1:

$\begin{matrix}{{\hat{p} = {\underset{p}{argmin}{\sum\limits_{i^{\prime} = {- n}}^{n}{\sum\limits_{j^{\prime} = {- n}}^{n}{\sum\limits_{k^{\prime} = {- n}}^{n}{\left( {V_{{i + i^{\prime\;}},{j + j^{\prime}},{k + k^{\prime}}} - {M_{i^{\prime},j^{\prime},k^{\prime}}(p)}} \right)^{2}w_{i^{\prime},j^{\prime},k^{\prime}}^{2}}}}}}},} & {{Equation}\mspace{14mu} 1}\end{matrix}$wherein for a positive integer n, indices i′, j′ and k′ take values from−n to n, p represents a free parameter vector, M_(i′,j′,k′)(p)represents a model value for the (i+′,j+j′,k+k′) voxel in the volume,w_(i′,j′,k′) represent weight factors and {circumflex over (p)}represents the optimal value of the free parameter.

The weight factors can be determined based on Equation 2:w _(i′,j′,k′) =w ^(spatial) _(i′,j′,k′) w ^(HU) _(i′,j′,k′),  Equation2:wherein w^(spatial) _(i′,j′,k′) represents weights for neighboringvoxels according to their spatial distance to V_(i,j,k), and w^(HU)_(i′,j′,k′) represents weights for neighboring voxels according to theirvalue-distance to V_(i,j,k) in the HU (Hounsfield Unit) space.

w^(spatial) _(i′,j′,k′) can be determined based on Equation 3:

$\begin{matrix}{{w_{i^{\prime},j^{\prime},k^{\prime}}^{spatial} = \sqrt{\exp\left( {- \frac{\left( {\left( {i^{\prime}{dx}} \right)^{2} + \left( {j^{\prime}{dx}} \right)^{2} + \left( {k^{\prime}{dz}} \right)^{2}} \right)}{2\sigma_{spatial}^{2}}} \right)}},} & {{Equation}\mspace{14mu} 3}\end{matrix}$wherein dx is the size of the pixel (e.g., in millimeters, or mm), dz isthe slice width (e.g., in mm), and σ_(spatial) is an algorithm parameterthat controls the aggressiveness of the weights.

w^(HU) _(i′,j′,k′) can be determined based on Equation 4:

$\begin{matrix}{{w_{i^{\prime},j^{\prime},k^{\prime}}^{HU} = \sqrt{\exp\left( {- \frac{\left( {V_{i,j,k} - V_{{i + i^{\prime}},{j + j^{\prime}},{k + k^{\prime}}}} \right)^{2}}{2\left( {{\hat{n}}_{i,j,k}{mult}} \right)^{2}}} \right)}},} & {{Equation}\mspace{14mu} 4}\end{matrix}$wherein {circumflex over (n)}_(i,j,k) is the local noise level estimateof voxel V_(i,j,k) and mult is an algorithm parameter that control theaggressiveness of the weights. The specific localization weightingdirects the fitted models to preserve the local structure of the volume.

For explanatory purposes, assume that the model bank 406 includes twomodels 404, a constant model M_(i′,j′,k′)(c)=c and a second orderpolynomial model

${M_{i^{\prime},j^{\prime},k^{\prime}}\left( {a,b,c,d} \right)} = {{a\;\frac{i^{\prime\; 2}}{n^{2}}} + {b\;\frac{j^{\prime\; 2}}{n^{2}}} + {c\;\frac{k^{\prime\; 2}}{n^{2}}} + {d.}}$Of course, other models are contemplated herein.

For the constant model, the model fit coincides with a weightedaveraging as a function of shown in Equation 5:

$\begin{matrix}{{{\hat{V}}_{i,j,k}^{1} = \frac{\sum\limits_{i^{\prime} = {- n}}^{n}{\sum\limits_{j^{\prime} = {- n}}^{n}{\sum\limits_{k^{\prime} = {- n}}^{n}{V_{{i + i^{\prime}},{j + j^{\prime} + k + k^{\prime}}}w_{i^{\prime},j^{\prime},k^{\prime}}^{2}}}}}{\sum\limits_{i^{\prime} = {- n}}^{n}{\sum\limits_{j^{\prime} = {- n}}^{n}{\sum\limits_{k^{\prime} = {- n}}^{n}w_{i^{\prime},j^{\prime},k^{\prime}}^{2}}}}},} & {{Equation}\mspace{14mu} 5}\end{matrix}$wherein {circumflex over (V)}¹ _(i,j,k) represents the new noiselessestimate for voxel V_(i,j,k). Equation 5 can be obtained using astandard minimizing technique, for example, by finding the zero of thefirst derivative on a cost function.

For the second order polynomial model, the model fit coincides with theleast squares solution {right arrow over (x)}=(d,a,b,c) of theover-determined system of linear equations as represented in Equation 6:A{right arrow over (x)}={right arrow over (b)},  Equation 6:wherein the matrix A is a (2n+1)³×4 matrix with rows

${\begin{bmatrix}1 & \left( \frac{i^{\prime}}{n} \right)^{2} & \left( \frac{j^{\prime}}{n} \right)^{2} & \left( \frac{k^{\prime}}{n} \right)^{2}\end{bmatrix}w_{i^{\prime},j^{\prime},k^{\prime}}},$for a fixed integer triple (i′,j′,k′), and the vector b is a vector oflength 2n+1 with corresponding entries V_(i+i′,j+j′,k+k′)w_(i′,j′,k′)for each fixed integer triple (i′,j′,k′). The least squares solution ofEquation 6 can be found based on Equation 7:{right arrow over (x)}=(A ^(T) A)⁻¹ A ^(T) {right arrow over(b)}.  Equation 7:The inverse of matrix A^(T)A can be found using various matrix inversionalgorithms.

For explanatory purposes, assume that the matrix A^(T)A has a full rankin order. Also assume that the first component of the vector {rightarrow over (x)} (value d) is the component of interest since only thecentral pixel of the n-neighborhood is being updated and the estimatednoiseless value of the voxel V_(i,j,k) is {circumflex over (V)}²_(i,j,k)=M_(0,0,0)({right arrow over (x)})=d.

Under the above assumptions, the noiseless estimate can be determinedbased on Equation 8:

$\begin{matrix}{{{\hat{V}}_{i,j,k}^{2} = \frac{{\begin{bmatrix}C_{11} & C_{21} & C_{31} & C_{41}\end{bmatrix} \cdot A^{\prime}}b}{{A^{\prime}A}}},} & {{Equation}\mspace{14mu} 8}\end{matrix}$wherein C_(ij) is the (i, j) cofactor of matrix A′A and [C₁₁ C₂₁ C₃₁C₄₁] is a 1×4 matrix consist of four cofactors.

In one instance, one of the above two models, such as the simplest ofthe two models, is fitted to the image data first. In this example, theconstant model is fitted first. A metric is then determined for thefitted model. An example of a suitable metric is described in Equation9:

$\begin{matrix}{\frac{{local}\mspace{14mu}{STD}\mspace{14mu}{in}\mspace{14mu}\left( {i,j,k} \right)\mspace{14mu}{over}\mspace{14mu}{\hat{V}}^{1}}{{local}\mspace{14mu}{STD}\mspace{14mu}{in}\mspace{14mu}\left( {i,j,k} \right)\mspace{14mu}{over}\mspace{14mu} V},} & {{Equation}\mspace{14mu} 9}\end{matrix}$wherein {circumflex over (V)}¹ is the resulting noiseless estimatedvolume of the first model and V is the original unprocessed volume.

The metric is compared against a predetermined threshold TH, forexample, as shown in inequality of Equation 10:

$\begin{matrix}{\frac{{local}\mspace{14mu}{STD}\mspace{14mu}{in}\mspace{14mu}\left( {i,j,k} \right)\mspace{14mu}{over}\mspace{14mu}{\hat{V}}^{1}}{{local}\mspace{14mu}{STD}\mspace{14mu}{in}\mspace{14mu}\left( {i,j,k} \right)\mspace{14mu}{over}\mspace{20mu} V} > {{TH}.}} & {{Equation}\mspace{14mu} 10}\end{matrix}$The threshold TH can be estimated using a Monte Carlo technique, forexample, over a synthetic homogeneous noisy study. The default value forthreshold TH can be variously set, for example, based on a statisticalsignificance greater than 50%, like 60%, 80%, 90%, 95%, 99% or otherstatistical significance.

If the metric satisfies the threshold, then the polynomial model isselected and applied. Otherwise, the constant model is selected andapplied. Note that Equation 10 captures the effectiveness of the firstmodel (the constant model in this example). In general, if the firstmodel is not effective enough, then the second model is employed.

Optionally, the above method can be applied iteratively. With aniterative approach, the result of one iteration is used as an input fora subsequent iteration. The subsequent iteration may further reducenoise.

The image quality enhancer 128 optionally includes a structuralcomponent enhancer 208. In one instance, the structural componentenhancer 208 provides adaptive structural component image enhancement.In one embodiment, the structural component enhancer 208 adaptivelysharpens the image data by incorporating the noise level estimate with asoft thresholding technique to target predetermined aesthetical imagesharpness.

The sharpening can be achieved using a Laplacian or other approach. Witha Laplacian approach, a level of sharpening can be determined adaptivelyas a function of a local Laplacian, local noise level, and various inputparameters. The parameters can be used to control an amount ofaggressiveness and adaptation of the sharpening.

The image quality enhancer 128 further includes an enhanced imagegenerator 210. The illustrated enhanced image generator 210 generates anenhanced image based on the image data and the noise reduced image data.In one instance, the enhanced image generator 210 generates the enhancedimage data by combining the image data and the noise reduced image dataon a voxel-by-voxel basis. The data can be combined based on a linear ornon-linear mixing algorithm.

In the illustrated embodiment, the data is combined based onpredetermined IQ (noise) criteria 212. The IQ criteria 212 areadjustable (e.g., can be set and changed based on user input), whichallows for controlling the noise level in the enhanced image data, whichis directly related to the dose of the acquisition. As such, the usercan set a noise level and generate enhanced image data, and then repeatthis for one or more other noise levels. In one instance, the IQcriteria 212 is set to allow for preserving the noise power spectrum.

The following describes one non-limiting approach for determining aratio mixing of the image data and the noise reduced image data. Thealgorithm takes standard deviation signals of the original and thefiltered image and forms another signal, the ratio of these two signals.The mix ratio is found as a median of that ratio after the outliers arethrown away.

Where I^(std) is the local standard deviation of the original image andI^(std2) is the local standard deviation of the denoised image, bothdetermined using the disk radius r≧1, their ratio can be calculated forevery pair of pixel indices (i, j) in the image, based on Equation 11:

$\begin{matrix}{{{S\left( {i,j} \right)} = \frac{I^{{std}\; 2}\left( {i,j} \right)}{I^{std}\left( {i,j} \right)}},} & {{Equation}\mspace{14mu} 11}\end{matrix}$(or

${S = \frac{I^{{std}\; 2}}{I^{std}}},$where the division is on an element by element basis). The first layerof outliers comes from the matrix O₁, which is defined as a dilatedversion of matrix B^(classifier) using the dilate radius d≧1, whereB^(classifier) is a Boolean matrix derived from the condition in theequation 10:

${B^{classifier}\left( {i,j} \right)} = \left\{ \begin{matrix}{{}_{}^{}{}_{}^{}} & {{{{if}\mspace{14mu}{S\left( {i,j} \right)}} > {TH}},} \\{{}_{}^{}{}_{}^{}} & {{otherwise}.}\end{matrix} \right.$Other outliers in matrix S are identified as those that are greater thanone. But instead of throwing those away, only their decimal part ischosen, as shown in Equation 12:S ₁={mod(S(i,j),1)| for all (i,j) such that O ₁(i,j)=‘false’},  Equation12:which can be written as S₁=mod(S(˜O₁), 1) in pseudo code notation. Theestimate factor f can be determined as the median of the signal S₁ or asthe mode of the signal S₁.

Given a targeted noise reduction T, the first mix ratio estimate is thendetermined from the requirement that the noise variance of the processedimage is equal to the variance of the original image multiplied by 1−Tbased on in Equation 13:

$\begin{matrix}{p_{1} = {1 - {\frac{1 - \sqrt{1 - T}}{1 - f}.}}} & {{Equation}\mspace{14mu} 13}\end{matrix}$

In a next step, the mix ratio p₁ is corrected by the multiplier k andbias b based on Equation 14:p ₂ =k*p ₁ +b.

Then, the mix ratio p is determined from the requirement that it must bewithin a predetermined range 0≦m₁<m₂,≦1, for example, based on theformula in Equation 15:

$\begin{matrix}{p = \left\{ \begin{matrix}{m_{1},} & {{{{if}\mspace{14mu} p_{2}} > m_{1}},} \\{m_{2},} & {{{{if}\mspace{14mu} p_{2}} < m_{2}},} \\{p_{2},} & {{otherwise}.}\end{matrix} \right.} & {{Equation}\mspace{14mu} 15}\end{matrix}$

As discussed above, the image quality enhancer 128 as described hereincan be used to enhance lower dose image data so that the image qualityof the lower dose image data is the same or substantially similar to theimage quality of full dose image data.

This is graphically illustrated in FIG. 5, which shows gradientmagnitude distribution 502, 504, and 506 respectively for a full (100%)dose scan, a 50% dose scan, and a 50% dose scan with image enhancement.The magnitude distribution is proportional to image quality. As shown,the gradient distribution 506 for the 50% dose scan with imageenhancement is substantially similar to the gradient distribution 502for the full (100%) dose scan.

Variations are contemplated.

In FIG. 2, the noise modeler 204 generates a noise STD distribution,based on image data, that is used to generate the enhanced image data.In an alternative embodiment (FIG. 6), the noise modeler 204 generates anoise variance distribution, based on projection data, that is used indetermining the enhanced image data.

With this example, the noise modeler 204 processes the line integralmeasurements in the projection data and generates a noise variancedistribution for the projection data. In one instance, this can beimplemented as follows. First, a variance sinogram is generated from theprojection data. For this purpose, for each line integral value (or eachsample in the sinogram), the variance of the measurement is computed asa function of Equation 16:

$\begin{matrix}{\frac{1}{\left( {N\; 0} \right) \cdot {\mathbb{e}}^{({- P})}};} & {{Equation}\mspace{14mu} 16}\end{matrix}$where N0 is the number of input photons to the patient along the currentray and P is the line integral of the attenuation along the current ray.

Equation 16 takes into account the Poisson nature of thephoton-emissions. In other embodiments, Equation 16 can be extended toalso take into account noise from scatter, beam hardening, electronicnoise, and/or other effects. An example of a model that also takes intoaccount other effects include the model described in “Scatteredradiation in cone-beam computed tomography: analysis, quantification andcompensation,” Jens Wiegert, PhD Thesis, 2007.

In an alternative embodiment, a new sinogram, which can be generated byforward-projecting through the volume of reconstructed image images in2D or 3D, is used instead of the original projection data. As such, thenoise variance distribution can be generated even when the originalprojection data is not available or readably accessible.

The noise modeler 204 processes the noise variance distribution andgenerates noise variance image data. The noise modeler 204 can employ areconstruction algorithm similar to an algorithm employed by thereconstructor 120 of FIG. 1 or a different reconstruction algorithm thatcan determine a noise variance for each voxel or a selected subportionof the voxels in the image data generated by the reconstructor 120.

By way of example, in one instance the noise modeler 204 reconstructsthe data in a manner identical or nearly identical to a conventionalfiltered backprojection reconstruction, requiring two convolutionoperations (instead of only one), and a modified view-weighting functionin the case of 3D backprojection.

In instances in which the projections are interpolated in the angularand radial directions (angular and radial rebinning), the interpolationmay impact the noise in the reconstructed image data. The abovealgorithm is also compatible with methods that incorporate the effect ofthe angular and radial rebinning, for example, a multiplication of thevariance sinogram data by a factor that varies spatially across thedetectors.

It is to be appreciated that the above discussed noise variance imagedata can also be used with other applications. By way of example, thenoise variance image data can be used with an image-denoising algorithmto facilitate noise removal.

In one instance, the square root of this noise variance image can beused in the noise removal step 206 instead of the result of the noisemodeler 204 (see FIG. 2).

In other instance, this includes applications employing theRudin/Osher/Fatemi noise reduction algorithm. For example, given a noisyimage f(x, y), its domain Ω and scalar λ, this algorithm can be used toobtain the noiseless image u(x, y) as the minimizer of the costfunction:

${\left( \min \right){\int_{\Omega}{{{\nabla u}}{\mathbb{d}x}{\mathbb{d}y}}}} + {\frac{\lambda}{2}{\int_{\Omega}{\left( {f - u} \right)^{2}{\mathbb{d}x}{{\mathbb{d}y}.}}}}$With this cost function, the image noise variance can be used to varythe strength of the λ parameter, proportionally to 1/v, where v is theestimated image variance (per voxel).

In other embodiments, other algorithms are used to perform theimage-de-noising operation, for example, other algorithms that reduceimage noise while preserving strong gradients of real structures andincorporate the noise variance data into the de-noising process. By wayof another example, the noise variance image data can be used withapplications for optimizing noise, such as optimizing noise in view ofdose. Such an application is described in patent application Ser. No.61/253,881, filed Oct. 22, 2009, and entitled “Acquisition ProtocolAssessment Apparatus,” which is incorporated in its entirety herein byreference.

FIGS. 7-11 describe various methods. It is to be appreciated that theordering of the acts is non-limiting and one or more of the acts mayoccur in a different order. In addition, one or more of the acts may beomitted and/or one or more acts may be added.

FIG. 7 illustrates a method for enhancing image data. As discussedherein, this includes enhancing low dose image data so that the imagequality of the low dose image data is the same or substantially similarto the image quality of full dose image data.

At 702, a noise distribution is modeled. In one instance, the noisedistribution is a noise STD distribution generated based image data. Inanother instance, the noise distribution is a noise variancedistribution generated based projection data.

At 704, noise reduced image data is generated by removing noise from theimage data based on noise STD distribution or the noise variancedistribution.

At 706, the structure in the noise reduced image data is enhanced. Inanother embodiment, act 706 is omitted.

At 708, enhance image data is generated based on the initial image dataand the noise reduced image data, for example, by combining the initialimage data and the noise reduced image data based on an image qualitythreshold as described herein.

FIG. 8 illustrates a method for modeling noise in image data.

At 802, a noise standard deviation is determined for the image data.

At 804, a first set of outliers are identified in the image data basedon the noise standard deviation and a global algorithm.

At 806, noise reduced image data is determined based on a set ofinliers, which are determined based on the first set of outliers. Asdiscussed above, this may be achieved by applying a smoothing filterover the inliers.

If it is determined that the noise images data should be refined, thenat 808 a subsequent set of outliers is identified based on the set ofinliers and a refined noise reduced image data is determined based on asubsequent set of inliers, which are determined based on the subsequentset of outliers.

FIG. 9 illustrates a method for modeling noise in projection data.

At 902, a variance distribution is generated based on the projectiondata.

At 904, first filtered variance image data is generated by filtering thevariance distribution using a first filter. An example first filter is aramp squared filter.

At 906, second filtered variance image data is generated by filteringthe variance distribution using a second filter. An example secondfilter is a shifted ramp filter.

At 908, the first and second filtered variance data is combined. In onenon-limiting example, the data is combined using a stretch-combinealgorithm such as the following:v(r,z)=w²v(r,s₀)+(1−w)²v(r,s₀+1)+2w(1−w)v(r.s₀).

At 910, noise variance image data is generated by backprojecting thecombined data. This can be done using squared wedge or other weights, orno weights.

FIG. 10 illustrates a method for removing noise from image data andgenerating noise reduced image data.

At 1002, one or more models are fitted to the image data.

At 1004, one of the one or more models is identified likely to removenoise from the image data while preserving the structure representingthe scanned object or subject in the image data.

At 1006, the identified model is applied to the image data to removenoise from the image data and generate the noise reduced image data.

FIG. 11 illustrates a method for generating enhanced image data.

At 1102, the initial image data is obtained.

At 1104, the noise reduced image data is obtained.

At 1106, the enhanced image data is generated based on the initial imagedata and the noise reduced image data. As discussed herein, thepercentage of each of the initial image data and the noise reduced imagedata contributing to the enhanced image data is based on predeterminedIQ criteria.

The above may be implemented by way of computer readable instructions,which when executed by a computer processor(s), cause the processor(s)to carry out the described acts. In such a case, the instructions arestored in a computer readable storage medium associated with orotherwise accessible to the relevant computer.

The invention has been described herein with reference to the variousembodiments. Modifications and alterations may occur to others uponreading the description herein. It is intended that the invention beconstrued as including all such modifications and alterations insofar asthey come within the scope of the appended claims or the equivalentsthereof.

What is claimed is:
 1. A method, comprising: modeling a noisedistribution based on image data by determining a noise standarddeviation for sub-portions of voxels in the image data, wherein thesub-portions include outliers corresponding to non-homogeneous regionsand inliers corresponding to homogenous regions; generating reducednoise image data based on the noise distribution by fitting variouslocal structural models to the image data, selecting a model from thefitted models that is likely to remove noise from the image data whilepreserving the structure representing a scanned object or subject in theimage data, and applying the selected model to the image data to removenoise; and generating enhanced image data based on the image data andthe noise reduced image data by combining the image data and the noisereduced image data on a voxel-by-voxel basis based on a linear ornon-linear mixing algorithm and predetermined image quality noisecriteria, wherein an image quality of the enhanced image data issubstantially similar to an image quality of higher dose image data. 2.The method of claim 1, further comprising: generating the enhanced imagedata based on noise reduced image data and the lower dose image data. 3.The method of claim 2, further comprising: analyzing one or more dataacquisition parameters used to scan an object or subject and generatethe image data and identifying information from the one or more dataacquisition parameters.
 4. The method of claim 3, further comprising:using the information when determining spatial kernel weights for noiseremoval.
 5. The method of claim 4, the information comprising at leastone of a minimum number of spatial weights and a central spatial weightfor filtering kernel.
 6. The method of claim 1, further comprising:identifying the outliers in the image data based on a global algorithmand the noise standard.
 7. The method of claim 6, wherein the globalalgorithm includes: segmenting the scanned object or subject from therest of the image data; and analyzing the segmented data using ahistogram of the local standard deviation of small regions of thescanned object or subject in which the histogram represents thedistribution of the local standard deviation in the image data, whereinthe local standard deviation on suspected outliers are identified usingthe histogram.
 8. The method of claim 7, further comprising: processingthe inliers based on a smoothing algorithm, which is applied over localstandard deviation image data.
 9. The method of claim 8, wherein thepredetermined image noise criteria is user adjustable.
 10. The method ofclaim 1, further comprising: enhancing structural components in theimage data.
 11. The method of claim 1, wherein the enhanced image datais substantially invariant to a slice thickness of a data acquisitionresulting in the image data.
 12. The method of claim 1, wherein enhancedimage data and the higher dose image data have a substantially similarnoise variance.
 13. A system, comprising: a noise modeler that generatesa noise distribution for image data by determining a noise standarddeviation for sub-portions of voxels in the image data, wherein thesub-portions include outliers corresponding to non-homogeneous regionsand inliers corresponding to homogenous regions; a noise remover thatremoves noise from a lower dose image data based on the noisedistribution by fitting various local structural models to the imagedata, selecting a model from the fitted models that is likely to removenoise from the image data while preserving the structure representing ascanned object or subject in the image data, and applying the selectedmodel to the image data to remove noise, thereby generating noisereduced image data; and an enhanced image generator that generatesenhanced image data based on the lower dose image data and the noisereduced image data by combining the image data and the noise reducedimage data on a voxel-by-voxel basis based on a linear or non-linearmixing algorithm and predetermined image quality noise criteria whereinan image quality of the enhanced image data is substantially similar toan image quality of higher dose image data.
 14. The system of claim 13,wherein the noise distribution is a noise variance distributiondetermined from projection data used to generate the lower dose imagedata.
 15. The system of claim 13, further comprising: a structuralenhancer that adaptively sharpens structural component in the image databased on a local Laplacian, a local noise level, and a parameter thatcontrols an amount of aggressiveness and adaptation of the sharpening.16. The system of claim 13, further comprising: an acquisition parameteranalyzer that determines information from one or more data acquisitionparameters, which are used to generated enhanced image data, which issubstantially invariant to a slice thickness of a data acquisitionresulting in the image data.
 17. The system of claim 16, wherein theinformation includes at least one of a minimum number of spatial weightsor a central spatial weight for a filtering kernel.
 18. The system ofclaim 17, wherein the weights facilitate balancing a relative influenceof neighboring voxels in filtering kernel while keeping a size and astrength of a spatial component of the filtering kernel invariant to theslice thickness.
 19. The system of claim 13, wherein the enhanced imagedata has a gradient magnitude that is substantially similar to agradient magnitude corresponding to higher dose image data.
 20. A methodfor modeling noise in image data, comprising: determining, via aprocessor, a noise standard deviation for the image data; determining,via the processor, a first set of outliers, which include edgescorresponding to non-homogeneous regions, in the image data based on thenoise standard deviation and a global algorithm by segmenting structuresfrom the image data through thresholding and region growing, analyzingthe segmented data using a histogram of a local standard deviation ofsmall regions which histogram represents a distribution of the localstandard deviation in the image data, and identifying the outliers asvalues past a fixed percentage of the distribution; determining, via theprocessor, a set of inliers, which include non-edges corresponding tonon-homogeneous regions, based on the set of outliers; and determining,via the processor, a noise distribution based on the set of inliers,which are determined based on the first set of outliers.
 21. The methodof claim 20, further comprising: determining a second set of outliersbased on the set of inliers and a local algorithm; and refining thenoise distribution based on a second set of inliers, which aredetermined based on the second set of outliers.